The present invention relates to reactive polyurethane mixtures and to their use as one-component adhesive systems.
Heat-curing one-component reactive adhesives based on solid, finely divided polyisocyanates stabilized by coating with polyadducts and isocyanate-reactive compounds, such as polyols and polyamines, are known in the patent literature (see for example European patents 62,780, 100,508 and 153,579 and German Auslegeschriften 3,112,054, 3,228,723 and 3,403,499).
There is considerable interest in the bonding of motor vehicle body parts of, for example, glass-fiber-reinforced polyester resin (SMC). Adhesives currently used which achieve their final strength by application of heat, are not genuine one-component adhesives, but two-component adhesives with a very long pot life or mixtures which react via more than one stage.
In the bonding of SMC parts, the heat is normally applied to the parts to be bonded by metal heating jaws, which are adapted to the contour of the parts to be made and through which heated thermal oil flows, and is thus transmitted to the layer of adhesive. Heating by the heating jaw process is much quicker than heating in a circulating air oven where air not metal is used as the heat transfer medium. Short cycle times are thus possible. Hitherto standard two-component adhesives, in which the polyisocyanate at least is present as the continuous phase, continue to react slowly on their own, even at room temperature, after the first heat shock and removal of the heat supply until they reach their final strength.
Heat-curing one-component adhesives of the described type based on powder-form solid polyisocyanates require the continuous application of heat from outside to completely react. Accordingly, a hitherto unsolved problem in the bonding of SMC parts by the heating-jaw technique has been to allow the one-component adhesive to set to such an extent in the first hardening phase (for example 2 min. at 120.degree. C.) that good handling characteristics are obtained (hand strength &gt;1N/mm2) and the bond acquires its ultimate strength during the thermal after-treatment phase (paint oven).
The rate at which the adhesive is brought to the setting temperature is obviously crucial to the build up of cohesion (strength through high molecular build up) and adhesion (physical and chemical interactions of the adhesive boundary layer with the substrate surface).
In circulating air ovens, the heat is supplied relatively slowly. Accordingly, the temperature in the adhesive film rises relatively slowly and the reactants have sufficient time to react by diffusion and chemical reaction to form a high molecular weight substance with, at the same time, good adhesion to the substrate surface, i.e. to form a genuine adhesive.
In the heating-jaw process, heating is so rapid that, although the adhesive hardens, it is unable to develop any adhesion to the substrate surface. In cases such as these, even subsequent heat treatment is unable to produce the adhesion required for an adhesive.
Accordingly, the problem addressed by the present invention was to provide improved adhesives, particularly for rapid heating systems.